This invention relates to audio recording and, more particularly, to an apparatus and method for controlling the cutting of audio-modulated grooves on a recording disk.
A record or phonograph disk contains audio information cut into grooves which begin at the outer edges of the record and spiral inward toward the center. Although really one continuous groove, most people refer to the surface as having many individual grooves, and this convention will be followed herein. A fixed amount of the record surface is available for the audio-modulated grooves. The surface, in the case of a conventional 331/3 r.p.m. disk, is an annulus which extends from an outer diameter of 111/2 inches to an inner diameter of 43/4 inches. When cutting the record, an audio signal, typically music originating from a tape recorder, modulates a cutter which, in turn, causes modulation in the position and depth of the groove that is cut by the cutter in a lacquer master. The groove can be modulated vertically (so-called hill and dale) or laterally, or a combination of both. For a conventional stereo record, left and right channel signals are respectively applied to cause modulation of orthogonal cutter surfaces which intersect the record surface at 45.degree. angles, as will be illustrated below. This effectively causes both vertical and lateral modulation of the record groove in accordance with the difference and sum of the left and right channel signals.
Since the groove has a right-angle v-shaped cross section, the width of a groove is necessarily proportional to the depth of the groove. The groove width and the spacing between grooves determine the number of grooves which can be packed into the limited space on the record, and therefore determines the total playing time of the record. A greater degree of modulation, caused by louder signals, necessitates more modulation and wider grooves. To avoid cutting into an adjacent groove, provision must be made for increasing the inter-groove distance (or "pitch") for louder signals to avoid having the modulation peaks of adjacent grooves cut into each other. However, a provision for sufficiently wide constant pitch was found to yield grooves which were too far apart during quieter passages, this being wasteful of record area and resulting in a shorter total recording time for music of a given loudness. (In this respect, it can be noted that since loud audio requires more "space" on the record, a good measure of the total information which can be packed onto the existing record space is the product of the total record playing time multiplied by a measure of the loudness of the music being recorded. Accordingly, it is generally an objective to maximize the "time-loudness product" of the record.).
To reduce the waste of record space which results from constant pitch, automatic variable-pitch control systems have been devised to automatically increase pitch during loud passages and return to a lesser pitch during quieter passages. While improvements in time-loudness product have resulted from these techniques, applicants have noted that substantial record space is still wasted by existing techniques and further improvement would be most desirable. Existing techniques of variable-pitch control also have drawbacks which could stand improvement. For exmple, existing systems generally have an "advance" tape head which provides audio from the previous revolution of the recording media. This technique requires control over a sizable portion of audio tape. Also, in prior art techniques, pitch variations called for by automatic pitch control systems may themselves introduce undesirable audio distortions.
A further important consideration in cutting a record is the depth of the grooves in th record. The groove can be visualized as having a steady-state or "unmodulated" depth upon which the audio modulations are superimposed. This means that a very large audio signal will result in extreme alternating deep and shallow groove depths. To prevent the shallow excursions from becoming less than an established minimum groove depth needed for proper playback tracking, one can increase the unmodulated depth of the cutter. However, a countervailing consideration is than an unnecessarily deep modulated groove depth will result in a waste of record space since, as noted above, the groove width is proportional to its depth.
A still further consideration in control of the record cutting process is the existence of "echo" which is the undesirable transfer of modulation as between adjacent grooves. Echo is caused by a deformation of the wall between adjacent grooves and can be suppressed by providing, under certain conditions, extra room or "land" between adjacent grooves. However, once again a countervailing consideration is that provision of at least a minimum "land" can tend to be wasteful of record space on those portions of the record where the minimum land is not necessary.
The prior art contains various teachings which address one or more of the stated requirements and problems, for example the U.S. Pat. Nos. 2,611,038; 2,738,385; 2,847,514; 2,948,783; 2,963,556; 3,075,052; 3,223,789 and 3,850,436. In some cases, the teachings of the prior art are found to provide inadequate solutions to the problems set forth. For example, solution may be suggested to one of the stated problems while compromising or ignoring other important considerations. To applicants' knowledge, no prior comprehensive technique has been devised which takes into account problems pertaining to pitch, depth control, echo, and introduction of distortion by cutter head motion, while also optimizing the time-loudness product of a produced record.
It is an object of the present invention to provide comprehensive solution to the prior art problems as set forth and to generally effect enhancement of the accuracy and efficiency of cutting audio disk recordings.